Capacitive touchpad is an input device to control cursor movement by providing a smooth panel for user's finger or conductive object to touch or move thereon. Since capacitive touchpad is very thin, it can be designed into ultra-thin notebook, keyboard, digital player and other devices, and further, its non-mechanical design makes it very easy to be maintained.
FIG. 1 is a cross-sectional view of a conventional two-dimensional capacitive touchpad 100, which comprises a panel 102, a Y-axis sensing layer 104, an insulator layer 106, an X-axis sensing layer 108, and a bottom plate 110. When a finger 112 touches on the panel 102, the sensed value (in capacitance) on the touched position will have a variation, and the control circuit connected to the touchpad 100 can convert the capacitance on the touchpad 100 to a sensed value as shown in FIG. 2, by which the position where the finger 112 touches and the moving distance and the moving direction of the finger 112 can be determined. Conventionally, the sensed value on the touchpad 100 is used to determine if an object touches on the touchpad 100 by the way as shown in FIG. 3. When the sensed value is greater than a threshold th, it is determined that an object touches on the touchpad 100, and on the contrary, when the sensed value is less than the threshold th, it is determined that the object leaves the touchpad 100 or no object touches on the touchpad 100.
However, the detection for the touchpad 100 might be interfered by noise resulted from wireless device such as mobile phone and others, and therefore, the touchpad 100 might misjudge that an object touches, taps, moves or practices any gesture thereon. FIG. 4A is an example showing a noise generated on the touchpad 100, the noise will be converted to a sensed value by an analog-to-digital converter (ADC) as shown in FIG. 4B, and finally, by a sampling process, it will become a waveform as shown in FIG. 4C. The waveform of FIG. 4C is similar to one generated when two fingers touch on the touchpad 100 as shown in FIG. 5, and therefore, the noise exemplary shown in FIG. 4A could be misjudged to be an operation of two fingers touching on the touchpad 100, thereby causing the touchpad 100 to have an operation that is not desired or predicted.
Therefore, there is a need of a detection method for a touchpad that will avoid noise to interfere the operation of the touchpad.